Technical Excellence: Rising to the tunnelling challenges

The UK tunnellng industry appears secure, having positively embraced the introduction of new technology and processes and is now training a growing skilled workforce to help deliver complex projects such as Crossrail and the Lee Tunnel.

The challenge the industry now faces is to maintain that momentum and ensure that the experience and expertise continues to evolve.

For the first time in history, over half of the world’s population now lives in urban environments and this figure grows every week, according to the Holcin Foundation.

Among the many implications of this trend for future infrastructure and construction planning, and the need for new ideas and ways of engineering spaces, the crucially important role of the tunnelling sector worldwide is clear.

It has been an exciting time for the UK tunnelling industry, with construction on the Thames Tideway Tunnel about to start and major joint venture rail projects at various stages of procurement and construction. The profile of the industry has also been given a boost, helped in no small part by London’s Crossrail project and its starring role in the BBC documentary, “The £15bn pound railway”.

However, while it might be the one grabbing the limelight, Crossrail is just one project in a growing portfolio of schemes that form part of a strong pipeline of significant infrastructure opportunities.

“Tunnelling in the UK looks to have a bright future,” agrees Ferrovial Agroman business development director Grant Mobbs.

“There is an extensive pipeline of work on a variety of significant schemes. These include the Northern Line Extension, Thames Tideway Tunnel, Western Rail Access to Heathrow, Silvertown Tunnel, High Speed 2 and Crossrail 2.”

Over the next 10 years, this market is expected to continue to grow, but Mobbs has a note of caution.

“Techniques are continuously improving to include new ways of tunnelling, monitoring and construction”

Joaquin Horrillo, Ferrovial Agroman

“Clients need to collectively schedule their respective programmes on a broad industry scale to ensure contractors have the visibility and opportunity to plan their resource requirements.

The market will respond, providing it has clear visibility of client requirements and demands,” he says.

As the pipeline grows, so does the UK tunnelling industry’s reputation for leading the way. Recent projects bear testament to this.

“Techniques are continuously improving to include new ways of tunnelling, monitoring and construction. And the UK remains at the forefront, thanks to the large number of complicated tunnelling projects currently underway or which have been completed, particularly in London,” says Ferrovial Agroman head tunnelling advisor Joaquin Horrillo.

Health and safety culture

The construction industry is increasingly embracing aspects of social sustainability and corporate responsibility to its workforce and stakeholders and in tunnelling especially, this cannot be sidelined, says Horrillo.

“Health and safety is a major priority at Ferrovial Agroman and several examples of best practice, established on Crossrail and Heathrow Terminal 2A among others, are increasingly becoming commonplace. As well as protecting our people and others around our work sites, simple safety and well-being-focused changes can bring increases in productivity, efficiency and quality.

“One example of this is changing from a shift pattern of two 12 hour shifts to three eight hour shifts, and working seven days on, four days off. This is an approach which has been standardised across the industry, where 24 hour working is required.

“As the industry responds and adopts these examples of best practice, future projects will benefit, as will all those who work on them and live close to the sites.”

Horrillo believes there is a need for greater collaboration between contractors and tunnel boring machine (TBM) manufacturers, to ensure practical features are incorporated in plant specification.

“This will ultimately improve production rates, but more importantly the health, safety and well-being of the teams working above and below ground.”

Monitoring and instrumentation

A key area of improvement in the tunnelling sector in recent years has been the advance in modelling techniques and computing power, which means engineers are able to analyse and model increasingly large amounts of data, allowing more intelligent monitoring.

Crossrail has continued to provide a wealth of opportunities, for example in trialling new monitoring and instrumentation processes, many of which will become best practice.

“Monitoring and instrumentation is a key area where advances will be of great value to the sector globally in the coming years,” says Ferrovial Agroman head of technical, bidding department Sofia Guerrero.

“The tunnelling industry has been working with conventional instrumentation for some time, but there are many new types of sensor becoming available. Shape Accel Arrays (SAAs) for example, which were used for the first time on a tunnelling project on Crossrail. They are revolutionising working practices and will have a significant impact on the way the industry works in the future,” she explains.

“The industry needs to actively get involved with academia to help further technology and develop its approach in all areas”

Sofia Guerrero, Ferrovial Agroman

SAAs are sensors that can be placed in a borehole, or attached to or embedded in a structure, to monitor deformation.

They comprise a series of jointed segments, each with a triaxial accelerometer that can bend by up to 90°, allowing them to withstand large movements.

SAAs are installed horizontally and vertically in PVC ducts that are grouted in place. By measuring the changes in angles between the segments, and thereby determining the shape of the SAA, a detailed and highly accurate 3D picture of ground movements can be built up. And, perhaps most importantly for engineers working on complex underground structures, the resulting data can be sent wirelessly for real-time monitoring.

“Using SAAs in a tunnelling environment means that it is now possible to have an extremely accurate and reliable way of monitoring movements as tunnelling proceeds, which is particularly important on projects in urban areas, where the ground cover may only be two or three metres,” says Guerrero.

“The use of SAAs gives contractors trustworthy data that can be interrogated in real time - a huge advantage where you are tunnelling in and around other underground transport links, utilities and foundations,” she adds.

Research and development

Major projets: Lessons from large scale tunnelling can contractors improve value for money

While investment in new technology is vitally important, it is crucial that research and development work is carried out too, especially on major projects, which offer insights into complex tunnel construction in busy and sensitive environments.

“The industry needs to actively get involved with academia to help further technology and develop its approach in all areas, from infrastructure and intelligent cities to water treatment and energy efficiency,” says Guerrero.

“Ground movement caused by tunnelling and the effects on buildings in urban areas continues to be a major issue.

“We need to be developing and validating numerical deformation models for over consolidated clays, which are commonly encountered under major cities, such as London, Madrid and Barcelona, and analysing the deformation field produced by tunnel construction.

Risk of damage to assets

“Also, we need to improve how we assess the risk of damage to assets next to and directly above underground construction, particularly tunnels near the surface in congested areas. The aim should be to reduce the cost of expensive protective measures, such as compensation grouting or structural reinforcement, which must be designed and implemented before work can begin.”

These studies are already underway. Research students from the Massachussets Institute of Technology (MIT), for example, analysed data from Ferrovial Agroman’s work on Crossrail’s C300 Western Running Tunnels contract, resulting in the development of more soil constitutive models and 3D and finite element models for earth pressure balance tunnelling in London Clay.

Ferrovial Agroman’s head of geotechnics in its technical office Davor Simic says: “This research included incorporating non-linear stress-dependent properties of the clay into the constitutive model - features that are critical for an accurate evaluation of the ground movements which have a direct impact on the behaviour of existing assets.

“We can now use numerical tools that have been calibrated against recent tunnel construction to understand more accurately the settlement prediction for tunnels in London Clay.”

These partnerships between industry and academia, like the $5M (£3.2M), five year partnership signed in 2010 between Ferrovial and MIT, will deliver huge contributions to the underground construction sector as whole, Simic believes.

“Ultimately, this type of research which has given us a greater insight to ground behaviour; will enable more accurate settlement predictions to be made; prevent implementation of unnecessary costly and disruptive protective measures and allow more efficient designs to be developed. These are substituted by more efficient control of face pressure during the operation of the tunnel boring machine, when it comes to being close to sensitive structures.”

Other benefits include being able to evaluate the effectiveness of innovative solutions to ground movement control. This will help to give more confidence to third parties and asset owners about how they will be affected by tunnelling activity - a key consideration when the general public only see the disruption caused, rather than the long-term benefits.

Material advances

An area of innovation specified on TBMs includes an improved new weighing system to allow accurate spoil reconciliation between volume and weight of excavation in each advance.

“Sprayed concrete lining (SCL) tunnelling is likely to become one of the greatest influences on current and future tunnelling working methodologies,” Simic says.

The British Tunnelling Society (BTS) has recognised this and is drawing on the experience gained through the use of SCL on Crossrail to develop a good practice guide for the delivery of safe SCL works.

SCL is being used to construct 3.5km of station tunnels - platforms, concourses and cross-passages - at Bond Street, Tottenham Court Road and Farringdon stations, as well as in the Fisher Street crossover tunnel.

“Sprayed concrete lining (SCL) tunnelling is likely to become one of the greatest influences on current and future tunnelling working methodologies”

Davor Simic, Ferrovial Agroman

“These stations have some of the largest caverns ever to be built using SCL in London. The platforms are typically 11m wide by 10m high and 280m long,” says Simic.

“All of these permanent tunnels are under several listed buildings, critical assets, 3,500 stakeholders and under some of London’s most expensive real estate.

Compensation grouting

“Extensive compensation groutig through 45km of tubes-à-manchette provided a protective canopy for the buildings above.

“SCL requires constant feedback of information and continuous monitoring of its performance. For example, at Crossrail we had 11,000 monitoring points and 50 automated Total Stations across west London. Little-by-little, with every project, design and construction techniques are improving and our knowledge of SCL is expanding considerably. This knowledge adds greatly to the future potential use of SCL, in the UK and abroad,” Simic says.

“The SCL approach will really reap rewards and there is a huge knowledge bank in the UK right now. This knowledge should and could be used and exported to places like Hong Kong and Singapore, where the method is not currently used, but there is a lot of work in the pipeline that would benefit from its introduction.”

People power

As with most specialist sectors, technical excellence comes largely from the expertise and experience of the people working in tunnelling, and it is vital to retain existing engineers and ensure that a steady flow of new tunnellers continues.

In a recent interview in NCE, Roger Bridge, the new British Tunnelling Society (BTS) chairman, explained that the potential increase in workload for the wider construction industry as it comes out of recession presented problems in attracting qualified people to tunnelling.

“Capturing the attention and the imaginations of the tunnellers of the future is the key,” he says.

“UK PLC is already achieving high standards of broad civil engineering training for new entrants into the construction industry, but we need to give engineers exposure to the specific challenges associated with the delivery of complex tunnelling infrastructure projects.”

Horrillo agrees that there we are currently experiencing a skills shortage, given that it takes 10 years to take a young person from GCSEs to A Levels, through a degree and then into the workplace where they can get some experience.

“There are going to be pinch points in the skills pipeline. We need to attract more people into the industry, especially women, not only across the board, but particularly on the design side, to meet the demand we are likely to have going forward in the UK, with the Thames Tideway Tunnel, the Northern Line Extension and beyond.

“The industry doesn’t lobby hard enough to use the skills and solutions it has developed to encourage tunnelling as a solution to the infrastructure deficit outside London”

Grant Mobbs, Ferrovial Agroman

“It is something the industry is going to have to address as the skills base has already started to decline. There are a lot of people who are 55 to 60, but’s there’s going to be a gap before the next generation starts coming through.”

Certain initiatives have been set up to encourage this specialist training. The High Speed 2 project is establishing colleges to meet its potential needs and the Tunnelling and Underground Construction Academy was established in 2011 by Crossrail to address a shortage of apprentices with the necessary skills to work underground on the transport scheme and on other planned infrastructure projects.

Lobbying needed

“It is a start but I think more could be done and not just in London but on a wider global scale,” says Mobbs. “The industry doesn’t lobby hard enough to use the skills and solutions it has developed to encourage tunnelling as a solution to the infrastructure deficit outside London, particularly in Cardiff, Glasgow and Belfast, for example.”

Bridge says that courses like the MSc in tunnelling and underground space at Warwick University, will help provide undergraduate engineers with an understanding of the additional skills and also with some initial site experience that will assist them immediately when they arrive on site for their first role.

“The course at Warwick has seen a steady increase in the numbers attending - this figure has doubled in terms of full time students on the course this year from last year,” he says.

Others are calling for an expansion of the apprenticeship scheme, and for more sponsorship of school leavers, alongside what is on offer at the UK’s universities. The BTS, mindful of the industry’s need to improve its ability to attract new blood into the industry and support young people, has formed a young members committee, to enable members to work closely with schools and universities to spread the word.

The skills shortage could become more serious as the success and reputation for technical excellence of the UK tunnelling industry grows, particularly when it comes to retaining staff.

Infrastructure demand

In countries with growing populations, the demand for new infrastructure - from transport to water and waste water and energy supply - continues to grow. In rapidly expanding cities, where space is at a premium and there are increasingly challenging environmental constraints, the best way to meet these needs is to tunnel underground.

“The experience gained from Crossrail, and the use of new methods such as the SAAs and SCL, the progress and knowledge gained in instrumentation, monitoring and compensation grouting will make UK tunnellers very attractive to consultants and contractors based in the Middle East, Singapore and Hong Kong,” Mobbs points out.

“It is important this is acknowledged by all industry stakeholders, especially when it comes to recruiting. We are all drawing on the same employment pond, so securing the UK’s own pipeline of tunnelling work will be key to retaining engineering staff and the knowledge they have accumulated over the last few years on UK projects.

“It’s about realising there are several routes into the tunnelling profession but at the same time making sure we have exciting opportunities to offer. If all the work currently in the pipeline here in the UK comes to fruition, then that looks likely.”

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